The absence of estrogen exaggerates the effects of DHT on your hair, and androgenetic alopecia sets in.
An abrupt withdrawal
from tamoxifen therapy after, say, 3 months, will be followed by approx. 6 months of gradual hair loss. If your bad luck doesn't persist, your hair may begin to slowly normalize after that.
Aromatase inhibitors, such as arimidex, femara, and aromasin, are even harder on hair because they block the aromatase enzyme.